The present invention relates to an atomic layer deposition (hereinafter, referred to as “ALD”) apparatus and method for manufacturing a semiconductor device, and more particularly to an ALD apparatus and an ALD method for manufacturing a semiconductor device, in which a gas feed unit has an improved structure, thus enhancing the productivity of a depositing process, allowing gases to be uniformly supplied into a reactor, achieving the depositing process at a low temperature, and improving the physiochemical properties of an obtained thin film.
Generally, various thin films are deposited on a semiconductor substrate by a sputtering method, which is a physical deposition process. However, when the surface of the substrate is stepped, a thin film deposited by the sputtering method has low step coverage. Accordingly, a chemical vapor deposition (hereinafter, referred to as “CVD”) method using an organo-metallic precursor has been widely used.
Such a CVD method for depositing a thin film using a CVD apparatus has excellent step coverage of an obtained thin film and high productivity of the thin film deposition process. However, the CVD method has problems in that it requires a high temperature for depositing the thin film and has difficulty in finely controlling the thickness of the thin film to be several Å. Further, two or more kinds of reactive gases are simultaneously supplied into a reactor and chemically reacted with each other in a gaseous state, thus creating particles of contaminants.
Since semiconductor devices have been miniaturized recently, the thickness of thin films required by the semiconductor devices has decreased and it is necessary to finely control the thickness of the thin films. Particularly, in order to solve the above problems of the CVD method, there is proposed an ALD method, in which an atomic layer of an ultra-thin film is deposited to produce a dielectric layer of a semiconductor device, a transparent conductive layer of an LCD, or a protective layer of an electroluminescent thin film display.
In the above ALD method, the thin film is obtained by repeating cycles, in which reactants are separately injected into the reactor including a substrate (wafer), and chemically absorbed onto the surface of the substrate in a saturated state.
Hereinafter, the process and principle of the thin film deposition using the above ALD method will be described in detail.
FIGS. 1a to 1e are schematic views illustrating the ALD method, in which a thin film is obtained by repeating the ALD cycle twice.
First, a first reactive gas 12 is supplied onto the upper surface of a wafer 10 serving as a semiconductor substrate located within a rector. Here, the first reactive gas 12 is chemically absorbed onto the upper surface of the wafer 10 until the reaction reaches a saturated state (FIGS. 1a and 1b).
When the reaction between the first reactive gas 12 and the upper surface of the wafer 10 reaches the saturated state, the excess amount of the first reactive gas 12 no longer reacts with the upper surface of the wafer 10. Under this condition, an inert gas (not shown) reacts with the excess amount of the first reactive gas 12, thereby allowing the excess amount of the first reactive gas 12 to be exhausted to the outside (FIG. 1c).
After the first reactive gas 12 is completely removed from the reactor, a second reactive gas 14 is supplied onto the upper surface of the wafer 10, and then chemically absorbed onto the upper surface of the wafer 10. Here, the first and second reactive gases 12 and 14 are chemically reacted on the upper surface of the wafer 10, thereby being formed into a desired thin film of an atomic layer (FIG. 1d).
When the reaction between the second reactive gas 14 and the upper surface of the wafer 10 reaches a saturated state, the excess amount of the second reactive gas 14 does not react with the upper surface of the wafer 10 any more. Under this condition, an inert gas (not shown) reacts with the excess amount of the second reactive gas 12, thereby allowing the excess amount of the second reactive gas 12 to be exhausted to the outside (FIG. 1e).
The above-described steps shown in FIGS. 1a to 1e form one cycle, and the thin film of the atomic layer with a desired thickness can be grown on the wafer 10 by repeating the cycle.
In order to alternately supply the reactive gases, which are fed onto and chemically react with the upper surface of the wafer 10, a valve control unit is generally used.
FIG. 2 is a schematic cross-sectional view of a conventional ALD apparatus 20. Here, the movement of a wafer 24 is not shown.
The conventional ALD apparatus comprises a vacuum chamber 20 as a reactor, a base 22, for mounting the wafer 24 thereon, while moving upward and downward within the vacuum chamber 20, a gas suction port 26 installed at one end of the vacuum chamber 20, a gas exhaust port 28 installed at the other end of the vacuum chamber 20, and a gas feed unit 30 connected to the gas suction port 26. A heater is installed within the base 22.
Here, the gas feed unit 30 can comprise a first reactive gas container 32, a second reactive gas container 34, and a purge gas container 36 containing an inert gas depending on the types of thin films to be formed. First, second and third valves 37, 38 and 39 for controlling the flow rate of the corresponding gases are provided in the respective containers 32, 34 and 36.
In order to perform the steps shown in FIGS. 1a to 1e using the above ALD apparatus, a process cycle shown in FIG. 3 is carried out.
First, only the first valve 37 in the first reactive gas container 32 is opened so as to supply a first reactive gas into the vacuum chamber 20. When the absorption of the first reactive gas onto the wafer 24 is completed, the first valve 37 is closed and the third valve 39 is opened so as to supply a purge gas (inert gas) into the vacuum chamber 20.
After the first reactive gas is completely removed, the third valve 39 is closed and the second valve 38 in the second reactive gas container 34 is opened so as to supply a second reactive gas into the vacuum chamber 20. When the reaction between the second reactive gas and the wafer 24 is completed and a thin film is grown on the wafer 24, the second valve 38 is closed and the third valve 39 is re-opened so as to supply the purge gas into the vacuum chamber 20. Thereby, one cycle of the ALD process is completed. The thin film with a desired thickness is formed on the wafer 24 by repeating the cycle several times (FIG. 3).
The ALD process using the above conventional ALD apparatus is inevitably restricted by various requirements.
That is, a step of chemically reacting the second reactive gas with the first reactive gas absorbed onto the wafer must be performed in the low temperature range in which the first reactive gas is not dissolved on the substrate. In order to achieve the chemical reaction between the first and second reactive gases in such low temperatures, the second reactive gas must have a high reactivity, thus being limited in terms of materials to be selected.
For example, the second reactive gas with a high reactivity, which is used to form a metallic oxide thin film serving as a dielectric or electrode, is selected from vapor, ozone, etc. The use of these materials as the second reactive gas causes some unwanted problems.
Water is absorbed onto the inner wall of the reaction chamber and is not easily exhausted to the outside, thus lengthening the purging time and reducing the productivity of the ALD process. Further, water creates particles of contaminants, thereby deteriorating the uniformity and reliability of the obtained thin film. In case that ozone is used as the second reactive gas, since ozone has a high reactivity, it is difficult to induce ozone into the reaction chamber.
Further, ammonia (NH3) is mainly used as the second reactive gas to form a metallic nitride thin film. Ammonia is easily absorbed onto other parts other than the substrate within the reaction chamber, thus not being easily exhausted to the outside. In case that ammonia is not completely removed from the reaction chamber, ammonia reacts with a reactive gas of the next cycle, thus generating particles of a contaminant and increasing the amount of impurities in the thin film.
Moreover, with the ALD method, it is difficult to deposit a thin film made of a single element such as W, Al, Cu, Pt, Ir, Ru, etc. on the wafer.
Accordingly, in order to solve the above problems, there has been developed a plasma-enhanced ALD (hereinafter, referred to as “PEALD”) method.
In the PEALD method, a second reactive gas excited by plasma is supplied to a reaction chamber.
FIG. 3 is a graph illustrating gas supply in accordance with respective steps of the conventional ALD method.
First, a first reactive gas is supplied into the reaction chamber provided with a wafer, and absorbed onto the surface of the wafer. Then, a purge gas is supplied into the reaction chamber to remove the residual first reactive gas from the reaction chamber. Next, a second reactive gas excited by plasma generated by a plasma generator is supplied into the reaction chamber, and then acceleratedly reacts with the first reactive gas absorbed onto the substrate.
Thereafter, the plasma generation by the plasma generator is stopped and the supply of the second reactive gas is stopped. Then, a purge gas is re-supplied into the reaction chamber so that the purge gas reacts with the remaining amount of the second reactive gas, thereby removing the remaining amount of the second reactive gas from the reaction chamber.
Although the reactivity between the first and second reactive gases is low, since the second reactive gas of the PEALD method is excited by plasma, the reaction between the first and second reactive gases is easily achieved. However, in case that the plasma generator is turned on when the first reactive gas is supplied into the reaction chamber, the first reactive gas is dissolved, thus contaminating the particles of the first reactive gas or deteriorating the step coverage. Accordingly, the power supply to the plasma generator is synchronized with the gas supply cycle, such that the plasma generator is turned off when the first reactive gas is supplied into the reaction chamber, and turned on when the second reactive gas is supplied into the reaction chamber.
In order to supply the first reactive gas and the second reactive gas excited by plasma into the reaction chamber at a predetermined interval, the PEALD method employs a mode in which a plurality of valves are controlled by complicated steps. Such a complicated mode shortens the service life of the valves due to the frequent manipulation of the valves, and reduces the stability of the PEALD process due to the variation in the pressure in the reaction chamber caused by the disparity in the amount of the reactive gases and the purge gas supplied thereto.
That is, a PEALD apparatus used in the PEALD method comprises valves and a plasma system having a complicated structure and shortens the service life of the valves and the plasma system due to frequent manipulation, thereby increasing the maintenance cost and lengthening the shutdown time.
Further, the PEALD apparatus provided with the plasma generator does not comprise a mass flow controller (MFC) for electronically controlling the flow rates of the reactive gases, and has several problems such as the delay in time and speed required for manipulation of the valves.
Accordingly, since the flow rates of the reactive gases cannot be precisely controlled, the PEALD method has an unstable process.